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AMERICAN OPOSSUMS

155

course the chief interest of the genus, which may be " an un-
modified survivor from Mesozoic times, and therefore from a
time long before the Didelphyidae, Peramelidae., and Dasyuxidae
were differentiated one from the other." Another ancient feature

Fig. 82..

(found in Jurassic mammals) is a mylo-hyoid groove upon the
lower jaw, which, however, is not always present-, and its exist-
ence has therefore been denied. The single species, M. fasciatus,
is partly arboreal and partly terrestrial in habit, and feeds upon
ants. It is a Western and Southern Australian, form.

Fam. 2. DidelpJayidae*—All the members of this family are
pentadactylous. The
teeth are fifty in number,
arranged thus : I J C -J-
Pm -|- M -J-. The caecum
is small; the pouch is
generally absent ; the
tail generally long and
prehensile.

The genus Didelpliys
contains most of the
forms belonging to this
family, including as it
does some twenty-three
species. The Opossums
are mainly arboreal ani-

m als, insectivorous in   3T*Q« 83.—Virginian Opossum.   Didelphys virginiana*

,,     .    -      T      T     ,   ,,      ,                             x 4.    (After "Vogt and Specht.)

' their food ;  but the larger                                 *

species eat reptiles, birds, and their eggs. Several of the small
species carry their young, when able to leave tlie teats, <>n